Magnetic resonance imaging (MRI) device is a test that uses a magnetic field and pulses of radio wave energy to make images of organs and structures inside a body. Conventional MRI devices involve RF transmitters which are all operated on the basis of amplification of an analog input signal with only one high power RF power amplifier. A conventional MRI transmitter is illustrated in FIG. 1. However, studies are progressing to make multi-channel transmitters with lower power RF amplifiers for each channel and to take advantage of all-digital transmitter architectures. In this regard, new MRI transmitters are envisaged to include all-digital structures with multi-channel capabilities, RF low power amplifiers, and optical signal transmission. A new generation multi-channel MRI transmitter block diagram is illustrated in FIG. 2.
Via new generation multi-channel MRI transmitter with digital modulation method, each transmitter channel can be reconfigured individually. Many parameters like signal type, frequency of operation, phase and amplitude information for RF shimming can be changed easily from a control computer. High image quality with RF shimming capability is expected to be achieved with new generation multi-channel transmitter.
In brief, a direct all-digital transmitter uses a modulation technique to digitize and modulate analog signals. With modulation, the analog input signal is converted into a digital signal where the analog information is encoded into pulse width of digital pulses, which is referred to as pulse width modulation. After amplification of the digital pulses, digitally modulated pulses are then passed through a band pass filter to recover the analog information.
One of the critical components in the multi channel direct all-digital transmitter system is the modulator in which an up conversion to radio frequency is carried out. Delta Sigma Modulation (DSM) based IQ modulators are the most widely used components inside a direct all-digital MRI transmitter. DSM based digital IQ modulator architecture in literature is shown in FIG. 3. In a digital IQ modulator, an input signal is digitized and then decomposed into inphase and quadrature components. Next, these two signals are converted into 1-bit digital data with. Delta Sigma Modulator (DSM). DSM signals are multiplied by clock signals whose phase difference is 90° with xnor operation. Then I and Q arms are added in MUX. The clock frequency, fclock should be chosen at the desired MRI carrier frequency and the sampling frequency should be 4×fclock due to xnor and MU X operation. When the digital output signal is filtered with a bandpass filter at the clock frequency, modulated analog signal is recovered.
One drawback of the conventional DSM based IQ modulator is performing lower signal to noise ratio. The reason behind this is the possibility of feeding different amplitude levels in each DSM block at the I and Q arms of the modulator.
The present invention resolves this problem by using an IQ pre-modulation as shown in FIG. 5. Hence, the new amplitude levels at each DSM input are identical and the modulator performs better in SNR (Signal-to-Noise Ratio).
In the conventional digital IQ modulator shown in FIG. 3, only one MRI carrier frequency is generated. In MRI systems, multi-band MRI signal generation at fclock±deltaf (deltaf is order of Hz) is a critical process for reducing scan time. There are mixed-mode clock manager (MMCM) blocks in Field Programmable Gate Array (FPGA) to be used for clock generation. However with these blocks, clock generation with high frequency resolution (narrow frequency spacing in Hz), cannot be achieved due to the limited frequency generation capacity of digital phase-locked loop (PLL) structure inside the MMCM blocks.
Other technique shown in FIG. 1 to achieve multi-band frequency generation with high frequency resolution is direct digital synthesizer (DDS) modules. However, the output of a DDS module is analog which dictates digital to analog converter. Since, an analog MRI signal cannot be amplified with a highly efficient RF power amplifier (switch mode power amplifiers), system efficiency is limited for DDS module structures and an all-digital concept is violated.
Another technique conventional IQ modulator shown in FIG. 3 can be used to achieve multi-band frequency generation with high frequency resolution capabilities can be done at digital baseband. However, double side band modulation occurs as a result of IQ modulation. In FIG. 4, double side band modulation is seen at the output of the conventional IQ modulator. The unwanted band cannot be filtered before power amplification, because filtering operations convert the digital signal to an analog signal. Thus, highly efficient digital power amplification cannot be achieved and an all-digital transmitter concept is violated. Although the unwanted band can be filtered out at the power amplifier output with a high Q RF coil (see FIG. 3), power amplifier efficiency is reduced in half, since the unwanted hand of the double sideband is also redundantly amplified through the RF power amplifier.